Clinicians face a significant challenge in diagnosing and treating a medical condition characterized by transient symptoms, which may be influenced by changes in the disease state, patient activity, time of day, dietary influences, emotional influences, pharmaceutical side effects, etc. Diagnostic information available to a clinician is often limited to isolated clinical tests and examinations, which may or may not occur when a patient is symptomatic. Symptoms may gradually or suddenly worsen or improve over time making the physician's job in treating and diagnosing a condition even more difficult. Particularly in the field of cardiology, the mechanisms of such changes, for example in patients suffering from heart failure, are not fully understood. Acquisition of physiological data pertaining to a patient's medical condition on a continuous basis would be highly useful in diagnosing and treating individual patients and would provide valuable information for improving the medical understanding of disease mechanisms.
Ambulatory monitoring devices, such as a Holter monitor for studying a patient's ECG over an extended period such as 24 hours, are known. However, such ambulatory devices rely on patient compliance for collecting accurate data and are not generally tolerated for long periods of time such as weeks or months. Implantable medical devices are now available for acquiring and storing physiological data relating to a patient's medical condition over relatively long periods of time, such as one year or more. One such medical device is the Chronicle® Implantable Hemodynamic Monitor offered by Medtronic, Inc. The Chronicle device continuously senses a patient's EGM, intracardiac blood pressure signals, and an activity sensor signal and stores data in a looping memory whenever a data storage-triggering event occurs. Such data-storage triggering events may be related to a change in the patient's heart rhythm or a trigger delivered by the patient using an external hand-held device. The Chronicle device employs the leads and circuitry disclosed in commonly assigned U.S. Pat. Nos. 5,535,752 and 5,564,434, incorporated by reference herein, to record the EGM and absolute blood pressure values.
The patient may periodically download stored data to a home-based device for secure Internet transmission to Medtronic's Patient Management Network. A clinician may access the data at any time to view a continuous history of the cardiac data rather than the “snap shot” views normally obtained during office visits. Large amounts of physiological data collected during pertinent physiological or symptomatic events are made available to the clinician. The clinician, however, must now face the task of sorting through, analyzing and interpreting the large amounts of data, which may sometimes be an overwhelming task particularly in light of a clinician's large patient load.
Efficient recording of interesting aspects of physiological data has been desirable since ambulatory recording devices, both implantable and external, have first been used experimentally and clinically. Recording capabilities were introduced into implantable devices such as pacemakers and defibrillators after low power digital memory became implemented in these devices. Although the storage-to-size ratio of digital memory has increased with improved technology, storage of raw, uncompressed physiologic data, such as relating to ECG, blood pressure, oxygen saturation, body motion, respiration, blood flow, etc., is still not possible to achieve over a period of time longer than a day or so in small external, ambulatory recording devices and more particularly in small implantable devices.
Depending on the intended use, recordation of physiological data generally requires a sampling frequency of about 32 Hz or more. In some applications 100 Hz sampling or more is required. Approaches to reducing the memory requirements for storing physiological data include data compression methods, event-triggered data storage, data feature extraction methods, and data averaging.
Even when memory space is sufficient to store raw, uncompressed data, the data needs to be consolidated into a form that is usable and readily interpreted by the clinician. Consolidation of large amounts of stored data may be performed by an external computer after retrieval of the data from an implanted device. However, all-in-one storage and processing of data by an implanted device into a usable format that provides the physician with a clinically relevant view of the data would be more convenient than having to transfer and post-process data on another system.
As implantable device technologies improve, the available memory for storing physiological data has increased. Even so, the available memory within an implantable device is limited and therefore requires implementation decisions to be made regarding the time resolution of stored data and the length of a stored episode. If a detailed time resolution is desired, the duration of the recorded episode will be requisitely shorter. Physicians may desire to view varying resolutions of physiological data in order to analyze and understand the patient's condition over long-term, medium-term, and short-term trends. Having such data stored and immediately available by transferring the data from an implanted device to an external device can be convenient for the clinician in that the clinician is not required to download data from a remote website and match time and events of interest to large sets of data.
There remains a need, therefore, for a method and system for acquiring and storing physiological data in an efficient manner using a fixed amount of available memory in an implantable medical device or in an external ambulatory device having limited memory. Such a method preferably allows data to be stored in a way that permits a clinician to gain both long-term assessments of a patients disease status trends and detailed looks of recent or pertinent events.